1. Field of the Invention
The present invention relates to an impedance adjustment apparatus that is provided between a high frequency power source and a load, and that adjusts impedance seen from the high frequency power source to the load side.
2. Description of Related Art
FIG. 8 is a diagram showing an exemplary configuration of a high frequency power supply system. This high frequency power supply system is a system for performing a processing process such as plasma etching or plasma CVD on a workpiece such as a semiconductor wafer or a liquid crystal substrate, for example. The high frequency power supply system is constituted by a high frequency power source 1, a transmission line 2, an impedance adjustment apparatus 3, a load connecting portion 4, and a load 5 (plasma processing apparatus 5). The impedance adjustment apparatus may also be called an impedance matching apparatus. The high frequency power source 1 supplies high frequency power to the load 5 via the transmission line 2, the impedance adjustment apparatus 3, and the load connecting portion 4. In the load 5 (plasma processing apparatus 5), a plasma discharge gas is changed into a plasma state within a chamber (not shown) in which the workpiece is disposed, and the workpiece is processed using the gas that is in the plasma state. The gas in the plasma state is generated by introducing the plasma discharge gas into the chamber and supplying high frequency power from the high frequency power source 1 to an electrode (not shown) provided within the chamber to cause the plasma discharge gas to discharge.
With a plasma processing apparatus 5 that is used for applications such as plasma etching and plasma CVD, the state of the plasma constantly changes with the progress of the manufacturing process. The changing state of the plasma results in the impedance (load impedance) of the plasma processing apparatus 5 constantly changing. In order to efficiently supply power from the high frequency power source 1 to such a plasma processing apparatus 5, an impedance ZL seen from an output end of the high frequency power source 1 to the plasma processing apparatus 5 side (hereinafter, load-side impedance ZL) needs to be adjusted following a change in load impedance. For this reason, with the high frequency power supply system shown in FIG. 8, the impedance adjustment apparatus 3 is interposed between the high frequency power source 1 and the load 5 (plasma processing apparatus 5).
The impedance adjustment apparatus 3 is provided with elements having variable electrical characteristics such as variable capacitors or variable inductors. Variable capacitors are capacitors whose capacitance can be changed. The impedance adjustment apparatus 3 adjusts the load-side impedance ZL by adjusting electrical characteristics such as capacitance or inductance of the elements having variable electrical characteristics. The impedance adjustment apparatus 3 matches the output impedance of the high frequency power source 1 and the impedance of the load 5, by setting the electrical characteristics of the elements having variable electrical characteristics to suitable values. Matching the impedances enables the power supplied to the load 5 to be maximized by minimizing reflected wave power directed from the load 5 to the high frequency power source 1 as much as possible.
Because variable capacitors and variable inductors are elements whose electrical characteristics can be adjusted, in the present specification, variable capacitors and variable inductors are collectively referred to as “elements having variable electrical characteristics”. Also, information on capacitance, inductance and the like is referred to as “electrical characteristic information”.
FIG. 9 is a block diagram showing an exemplary configuration of a high frequency power supply system including a conventional impedance adjustment apparatus 3P.
A high frequency power source 1p is connected to an input end 301 of the impedance adjustment apparatus 3P by a transmission line 2, and a load 5 (plasma processing apparatus) is connected to an output end 302 by a load connecting portion 4. The high frequency power source 1p is a power source that outputs a high frequency wave having a constant output frequency. The output frequency is a fundamental frequency (frequency of a fundamental wave) of the high frequency wave that is output from the high frequency power source 1p. 
As shown in FIG. 9, the impedance adjustment apparatus 3P is provided with an adjustment circuit 20p constituted by a first variable capacitor 21, a second variable capacitor 24, and an inductor 23. The first variable capacitor 21 and the second variable capacitor 24 are one type of element having variable electrical characteristics. An output end of the adjustment circuit 20p is connected to the output end 302 of the impedance adjustment apparatus 3P, and a directional coupler 10 is provided between an input end of the adjustment circuit 20p and the input end 301 of the impedance adjustment apparatus 3P.
The high frequency power output from the high frequency power source 1p is supplied to the load 5 via the directional coupler 10 and the adjustment circuit 20p provided in the impedance adjustment apparatus 3P. Note that high frequency power that is output from the high frequency power source 1p and travels to the load 5 is called traveling wave power PF, and high frequency power that is reflected by the load 5 and returns to the high frequency power source 1p is called reflected wave power PR.
The impedance adjustment apparatus 3P is able to adjust (change) the load-side impedance ZL, by adjusting (changing) the capacitances of the first variable capacitor 21 and the second variable capacitor 24 provided in the adjustment circuit 20p. The impedance adjustment apparatus 3P matches the output impedance of the high frequency power source 1p and the impedance of the load 5, by changing the respective capacitances of the first variable capacitor 21 and the second variable capacitor 24 to suitable values. Note that the configuration of the adjustment circuit 20p differs depending on factors such as the output frequency of the high frequency power source 1p and conditions of the load 5. Variable inductors may be also used as elements having variable electrical characteristics.
The variable capacitors use for the first variable capacitor 21 and second variable capacitor 24 have a movable portion (not shown) for adjusting capacitance. The capacitance of the variable capacitors is adjusted by displacing the position of the movable portion using a motor or the like.
The variable capacitors are provided with a pair of electrodes, at least one of which is a movable electrode, with the movable electrode being the movable portion for adjusting capacitance. Because the size of the opposing area of the movable electrode and the other electrode changes when the position of the movable electrode is displaced, resulting in a change in capacitance, the capacitance of the variable capacitor is adjusted (changed) by adjusting (changing) the position of the movable electrode.
The capacitance of the variable capacitor is configured so as to be adjustable over a plurality steps. The capacitance relative to the position of the movable portion of the variable capacitor is known through the specifications of the variable capacitor or through testing. Because the capacitances of the variable capacitors are known if the position of the movable portion is known, position information of the movable portion is used as information representing the capacitances (capacitance information) in adjusting the capacitances of the variable capacitors. Accordingly, the position information of the movable portion of the variable capacitors is treated as information representing electrical characteristics of the variable capacitors (electrical characteristic information).
The position information of the movable portion of the variable capacitors may be any information obtained by directly or indirectly detecting the position of the movable portion. Because it is difficult given the structure of the movable portion to directly detect the position of the movable portion, the position of the movable portion is indirectly detected by, for example, detecting the rotation position (amount of rotation) of the motor that displaces the position of the movable portion. The rotation position of the motor can be detected using a pulse signal or voltage that controls the drive of the motor, or the like.
In the case of FIG. 9, the position of the movable portion of the first variable capacitor 21 is adjusted by an adjustment unit 30, and the position information of the movable portion of the first variable capacitor 21 is detected (acquired) by a position detection unit 40. Also, the position of the movable portion of the second variable capacitor 24 is adjusted by an adjustment unit 50, and the position information of the movable portion of the second variable capacitor 24 is detected (acquired) by a detection unit 60.
The adjustment unit 30 is a drive means for displacing the position of the movable portion of the first variable capacitor 21. The adjustment unit 30 is constituted by a stepping motor, a motor drive circuit and the like (all not shown), for example. The motor drive circuit provided in the adjustment unit 30 rotates the stepping motor based on a command signal that is input from a control unit 100p. The position of the movable portion of the first variable capacitor 21 is displaced by the rotation of the stepping motor. Accordingly, the control unit 100p adjusts the capacitance of the first variable capacitor 21 by controlling the amount of rotation of the stepping motor provided in the adjustment unit 30. Similarly, the adjustment unit 50 is a drive means for displacing the position of the movable portion of the second variable capacitor 24. The adjustment unit 50 is constituted by a stepping motor, a motor drive circuit, and the like (all not shown), for example. The motor drive circuit provided in the adjustment unit 50 rotates the stepping motor based on a command signal that is input from the control unit 100p, and displaces the position of the movable portion of the second variable capacitor 24. Accordingly, the control unit 100p adjusts the capacitance of the second variable capacitor 24 by controlling the amount of rotation of the stepping motor provided in the adjustment unit 50.
The position detection unit 40 detects the rotation position (amount of rotation) of the stepping motor provided in the adjustment unit 30. Similarly, the position detection unit 60 detects the rotation position (amount of rotation) of the stepping motor provided in the adjustment unit 50.
Note that variable inductors differ in structure from the variable capacitors, but also have a movable portion, similarly to the variable capacitors. The variable inductors are also similarly configured to be able to adjust (change) the inductance of the variable inductors by displacing the position of the movable portion using a motor or the like. Because the method used by the variable inductors to vary the inductance is basically the same as the variable capacitors, description thereof will be omitted. Because the inductances are also similarly known if the position of the movable portion of the variable inductors is known, in the case where the variable inductors are used as the elements having variable electrical characteristics, position information of the movable portion of the variable inductors is treated as information representing the inductances of the variable inductors (inductance information).
The first variable capacitor 21 and second variable capacitor 24 are configured to be able to adjust respective capacitances over a plurality of steps. For example, in the case where the position of the movable portion of the first variable capacitor 21 and the second variable capacitor 24 can each be displaced over 101 steps, the impedance of the adjustment circuit 20p can be changed in (101×101=) 10,201 (approx. ten thousand) combinations. That is, the impedance matching apparatus 3P is able to adjust (change) the load-side impedance ZL using approximately 10,000 impedance adjustment positions.
In the case where the position of the movable portion of the variable capacitors is displaced over a plurality of steps, allocating a number to each displacement position of the movable portion enables these numbers to be used as position information of the movable portion of the variable capacitors. For example, in the case where the position of the movable portion of the variable capacitors is displaced over 101 steps, assuming that the position at which the capacitance is minimized is “0” and the position at which the capacitance is maximized is “100”, the position information of the movable portion of the variable capacitor is represented by the numbers 0 to 100. Accordingly, assuming that the position information of the movable portion of the first variable capacitor 21 and the position information of the movable portion of the second variable capacitor 24 are each represented by the numbers 0 to 100, the impedance adjustment position of the impedance adjustment apparatus 3P is represented by position information that combines the position information of the movable portion of the first variable capacitor 21 and the position information of the movable portion of the second variable capacitor 24, such as (0,0), (0,1), . . . , (100,100).
For example, Patent Document 1 (JP 2006-166412A) proposes an impedance adjustment apparatus 3P that performs impedance matching by controlling elements having variable electrical characteristics such as variable capacitors or variable inductors.
With the impedance adjustment apparatus 3P disclosed in Patent Document 1, characteristic parameters of the impedance adjustment apparatus 3P that have been measured in advance are stored in a memory 70p. The characteristic parameters are parameters indicating transmission characteristics in the case where the entire impedance adjustment apparatus 3P is regarded as a transmission apparatus, and include, for example, S-parameters (scattering parameters) and T-parameters (transmission parameters) converted from the S-parameters. The characteristic parameters are measured with respect to all impedance adjustment positions of the impedance adjustment apparatus 3P (position information combining the position information of the movable portion of the first variable capacitor 21 and the position information of the movable portion of the second variable capacitor 24), after adjusting the impedance adjustment apparatus 3P to the respective impedance adjustment positions. Accordingly, the measured values of a plurality of characteristic parameters are stored in the memory 70p in association with the impedance adjustment positions. The control unit 100p then performs impedance matching, based on a detection signal of a traveling wave voltage and a detection signal of a reflected wave voltage that are output from the directional coupler 10, position information of the movable portion of the first variable capacitor 21 that is detected by the position detection unit 40, position information of the movable portion of the second variable capacitor 24 that is detected by the position detection unit 60, and information on the characteristic parameters stored in the memory 70p. 
Because the characteristic parameters are parameters indicating transmission characteristics within the impedance adjustment apparatus 3p including stray capacitance and inductance components, accurate impedance matching can be performed if impedance matching is performed using the measured characteristic parameters.
TABLE 1VC1012. . .. . .9899100VC20T (0, 0)T (1, 0)T (2, 0). . . . . .T (98, 0)T (99, 0)T (100, 0)1T (0, 1)T (1, 1)T (2, 1). . . . . . T (98, 1)T (99, 1)T (100, 1)2T (0, 2)T (1, 2)T (2, 2). . . . . . T (98, 2)T (99, 2)T (100, 2). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 T (0, 98) T (1, 98) T (2, 98). . . . . . T (98, 98) T (99, 98) T (100, 98)99 T (0, 99) T (1, 99) T (2, 99). . . . . .  T (98, 99) T (99, 99) T (100, 99)100 T (0, 100) T (1, 100) T (2, 100). . . . . .  T (98, 100) T (99, 100)T (100, 100)
Table 1 is an example of characteristic parameters stored in the memory 70p. Table 1 shows an example of the case where the characteristic parameters stored in the memory 70p are T-parameters. In Table 1, the position information of the movable portion of the first variable capacitor 21 is represented with a variable VC1, and the position information of the movable portion of the second variable capacitor 24 is represented with a variable VC2. Also, the variable range of the movable portion of the first variable capacitor 21 and the variable range of the movable portion of the second variable capacitor 24 are respectively ranges of 0 to 100 (101 steps).
In Table 1, T(0,0) indicates the T-parameter measured after adjusting the impedance adjustment apparatus 3P to an impedance adjustment position (0,0) (adjustment position at which the position information of the movable portion of the first variable capacitor 21 is “0”, and the position information of the movable portion of the second variable capacitor 24 is “0”). Similarly, T(100,0) indicates the T-parameter measured after adjusting the impedance adjustment apparatus 3P to an impedance adjustment position (100,0) (adjustment position at which the position information of the movable portion of the first variable capacitor 21 is “100”, and the position information of the movable portion of the second variable capacitor 24 is “0”). Other T-parameters are displayed with a similar approach. Note that although T-parameters are measured for all 10201 impedance adjustment positions of the impedance adjustment apparatus 3P, some of those values have been abbreviated as “ . . . ” in Table 1 in order to simplify the description.
Here, the S-parameters and the T-parameters will be described.
The S-parameters are parameters indicating the transmission characteristics of a prescribed four-terminal network (also known as a “two-port network”) at the time when a high frequency signal is input after connecting lines having characteristic impedance (e.g., 50Ω) to input and output terminals of the four-terminal network as is well known. The S-parameters are, as shown in Equation 1, represented as a matrix that is constituted by elements consisting of an input-side voltage reflection coefficient (S11), a transmission coefficient (S21) of a forward voltage, a transmission coefficient (S12) of a reverse voltage, and an output-side voltage reflection coefficient (S22)
                    [                                                            S                11                                                                    S                12                                                                                        S                21                                                                    S                22                                                    ]                            〈                  Equation          ⁢                                          ⁢          1                〉            
The T-parameters are, as shown in Equation 2, parameters that can be converted from the S-parameters. In measuring the transmission characteristics of a four-terminal network, the S-parameters are generally simple to measure, but the T-parameters are simple to use when performing calculations.
                                          1                          S              12                                ⁡                      [                                                                                                                              S                        12                                            ·                                              S                        21                                                              -                                                                  S                        11                                            ·                                              S                        22                                                                                                                                  S                    22                                                                                                                    -                                          S                      11                                                                                        1                                                      ]                          →                  [                                                                      T                  11                                                                              T                  12                                                                                                      T                  21                                                                              T                  22                                                              ]                                    〈                  Equation          ⁢                                          ⁢          2                〉            
In the four-terminal network shown in FIG. 10, the S-parameters are defined as in Equation 3 and the T-parameters are as defined as in Equation 4.
                              [                                                                      b                  1                                                                                                      b                  2                                                              ]                =                              [                                                                                S                    11                                                                                        S                    12                                                                                                                    S                    21                                                                                        S                    22                                                                        ]                    ⁡                      [                                                                                a                    1                                                                                                                    a                    2                                                                        ]                                              〈                  Equation          ⁢                                          ⁢          3                〉                                          [                                                                      b                  2                                                                                                      a                  2                                                              ]                =                              [                                                                                T                    11                                                                                        T                    12                                                                                                                    T                    21                                                                                        T                    22                                                                        ]                    ⁡                      [                                                                                a                    1                                                                                                                    b                    1                                                                        ]                                              〈                  Equation          ⁢                                          ⁢          4                〉            
In FIG. 10, the relationship between an input reflection coefficient Γin (input-end reflection coefficient) and an output reflection coefficient Γout (output-end reflection coefficient) when a port 1 is the input side and a port 2 is the load side can be represented using the S-parameters (see Eq. 5) or the T-parameters (see Eq. 6).
                                                                        Γ                ⁢                                                                  ⁢                out                            =                            ⁢                                                a                  2                                                  b                  2                                                                                                        =                            ⁢                                                                    Γ                    ⁢                                                                                  ⁢                    in                                    -                                      S                    11                                                                                                              S                      12                                        ·                                          S                      21                                                        +                                                            S                      22                                        ⁡                                          (                                                                        Γ                          ⁢                                                                                                          ⁢                          in                                                -                                                  S                          11                                                                    )                                                                                                                              〈                  Equation          ⁢                                          ⁢          5                〉                                                                                    Γ                ⁢                                                                  ⁢                out                            =                            ⁢                                                a                  2                                                  b                  2                                                                                                        =                            ⁢                                                                    T                    21                                    +                                                                                    T                        22                                            ·                      Γ                                        ⁢                                                                                  ⁢                    in                                                                                        T                    11                                    +                                                                                    T                        12                                            ·                      Γ                                        ⁢                                                                                  ⁢                    in                                                                                                          〈                  Equation          ⁢                                          ⁢          6                〉            
As described above, in the high frequency power supply system shown in FIG. 9, the output frequency of the high frequency power source 1p is fixed to a certain constant frequency. However, for example, in Patent Document 2 (JP 2006-310245), technology is proposed for performing impedance matching while adjusting the output frequency of the high frequency power source, focusing on the change in the load-side impedance seen from the output end of the high frequency power source to the load side when the output frequency of the high frequency power source is changed. With the technology described in Patent Document 2, impedance matching is performed by changing the output frequency of the high frequency power source to change the load-side impedance, because the capacitance component and the inductance component that are included in the load-side impedance change depending on the frequency. Note that, in the present specification, a high frequency power source whose output frequency can be adjusted (changed) in this way is called a high frequency power source 1v employing a variable frequency system.
Also, even in the case where a high frequency power source 1v employing a variable frequency system is used, an impedance adjustment apparatus that includes an adjustment circuit 20 shown in FIG. 11 is used in some cases, as described in Patent Document 3 (JP 2008-181846). The adjustment circuit 20 is an adjustment circuit in which the second variable capacitor 24 is replaced by a capacitor 22 having a fixed capacitance in the adjustment circuit 20p shown in FIG. 9. With the impedance adjustment apparatus described in Patent Document 3, impedance matching is performed by adjusting the output frequency of the high frequency power source 1v together with adjusting the position of the movable portion of the first variable capacitor 21. Note that because the capacitance of the capacitor 22 of the adjustment circuit 20 is fixed, an adjustment unit for adjusting capacitance and a position detection unit for detecting position information of the movable portion are not provided.
With the impedance matching method described in Patent Document 1 that performs impedance matching using characteristic parameters measured in advance, because of the characteristic parameters being measured with respect to a single output frequency, cases occur where impedance matching cannot be performed when the impedance matching method described in Patent Document 1 is applied to the high frequency power supply system described in Patent Document 2 or Patent Document 3 while changing the output frequency of the high frequency power source 1v. 